SECTION 2.7
The Effects of Manipulating

Certain manipulating factors provide some control of the metal cutting characteristics. Some effects of these factors will be discussed herein. The results described have been derived from machining various carbon and stainless steels with sharp high-speed steel broach cutting tools.

SECTION 2.7.1
Velocity (machining speed)

Velocity affects temperature, which in turn affects the cutting process. At low velocities, the temperature at the tool point is below the recrystallization temperature of the material. As a result, work hardening in the chip is retained and the workpiece material is not softened due to failure to reach the yield strength temperature of the material. If the velocity increases to the point where the cutting temperature is above the yield strength temperature of the material, the chip at the interface tends to soften and machine much more efficiently. Higher shear angles occur at higher velocities and an ideal chip thickness of 1.5 times the depth of cut can be approached. Excessive velocity will cause the tool to fail rapidly since speed has the greatest effect on tool life.

Chip form and shape at high velocities can be very troublesome on ductile materials. The reduced resistance to chip flow and the resultant increase in shear angle gives a thinner, less distorted chip, but one which becomes longer and straighter as velocity increases. Tooth gullets and geometries can be employed to deal with this problem, however.

SECTION 2.7.2
Depth of cut

Changes in the depth of cut effectively change the cross-sectional area of chip contact. How this area is changed determines the effect upon the cutting process. An increase in depth of cut widens the area of contact on the tooth face and changes the force. This results in greater chip distortion and reduced tool life, although increased depths reduce the machining cycle.

Several factors affect surface quality to a greater degree than may be predicted. Lack of rigidity will permit greater deflections as a result of higher forces. Increases in depth of cut may then cause chatter, poor surface quality, and loss of dimensional stability. Deep cuts on relatively small diameters might result in erratic tool life behavior and poor surface quality. The effect upon the chip is more pronounced with increases in the chip width than with increases in the chip depth. Because of the greater distortion caused by wider chips, width control (by the use of "chip breakers") is essential.

SECTION 2.7.3
Tooth geometry

For given cutting conditions, changes in tooth geometry have two direct effects on chip formation: (1) effect upon shear angle, and (2) effect upon chip thickness. The two are related in that a change in one usually affects the other.

The effects of changes in face (hook) angles are the most apparent. The lower hook angles decrease the shear angle, cause greater chip distortion, and increase the resistance to chip flow. Lower (and negative) hook angles produce rougher and more work-hardened surfaces. At low or negative hook angles, the chip is so highly distorted that it is sometimes broken into short lengths.

Figure 2-2.
A view of standard broach tooth geometry.

SECTION 2.7.4
Tool material

One effect of tool material lies in its ability to sustain high cutting velocities, as for example between high-speed steel and carbides. The effect of high velocity has already been described. Another factor is the coefficient of friction between chip and tool material. Usually, this is of little consequence with high-speed tools because the coefficient of friction does not change appreciably among the various grades.

SECTION 2.7.5
Cutting fluids

Ideally, if a cutting fluid provides lubrication between the chip and the tool, the coefficient of friction will be reduced and the shear angle will be increased. However, effective lubrication may be difficult to achieve except possibly at very low cutting speeds. Lubricant effects vary with cutting conditions and with work materials.

At high-speed, fluids act principally as coolants, but may effectively lubricate the tool-chip interfacial zone providing more efficient machining which often results in increased tool life and improved surface finish. Constant, even flow is essential when cutting fluids are applied with carbide tools to prevent thermal shock and resultant fractures. (For a fuller treatment of the subject of cutting fluids, see Section 3.0.)

SECTION 2.7.6
Workpiece materials

Brittle materials form discontinuous chips, and can be broached with decreased hook angle and no chip breakers. Cutting forces are usually lower for cast iron (and other high carbon steels) than they would be for a ductile material of corresponding strength because of generally large shear angles and lower resistance along the tool face.

Ductile materials produce continuous, curled chips. With low friction and high cutting velocities, particularly with material of low work-hardening capacity, a thinner, more tightly curled, less distorted chip is produced. Chip-breakers are required in order to break up the chip. High frictional resistance to flow, low shear angles, and materials of high work-hardening capacity are associated with large distortions during cutting, and are not as big a problem to break up the chip. Additions of lead, sulfur, and phosphorus to low-carbon steels help to break up chips, reduce built-up edge, and improve surface quality.